Donald Trump’s speaking slot at the Conservative Political Action Conference on Saturday is prompting an acrimonious backlash from the conservative critics desperately trying to mount a last-ditch campaign to block the GOP presidential front-runner from winning the party’s nomination.

A top aide to Trump rival Marco Rubio has accused CPAC organizers of being in the tank for Trump and clearing the way for his acceptance into mainstream conservatism, while an anti-Trump super PAC is pressuring organizers to rescind their invitation to the surging GOP front-runner.

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Potentially complicating matters further, sources tell POLITICO that Trump has made multiple donations totaling more than $100,000 ― including a $50,000 check last year ― to the American Conservative Union, the group that organizes CPAC. That dwarfs the amounts donated in recent years by allies of Trump’s rivals, all of whom are also scheduled to speak at the annual gathering, and seems likely to fuel already percolating suspicions among his opponents that the ACU has its thumb on the scale for Trump.

Organizers are trying to strike a tricky balance between their self-described role as neutral referees, and a leeriness about being seen as rewarding the increasingly incendiary campaign of the billionaire real estate showman threatening to shatter the Republican Party for years to come.

Ian Walters, an ACU spokesman, dismissed what he called “a concerted effort by some to try and make CPAC about Donald Trump,” explaining that his group has “no intention of diminishing the historic opportunity” for activists to hear from each of the candidates for the 2016 GOP presidential nomination.

While Walters would neither confirm nor deny Trump’s contributions, he said donations do not influence ACU’s decisions about who gets to speak at CPAC. “When someone supports us, it is them saying that we agree with ACU and CPAC’s mission,” said Walters. “There are over 120 partner organizations for this year’s CPAC, and it doesn’t affect stage time. That editorial control is ours, and we maintain that.”

Trump’s campaign manager, Corey Lewandowski, acknowledged that his boss “has donated to CPAC over the years,” but he added “nothing was asked for in return.” And Lewandowski seemed unconcerned over the backlash around the gathering, professing: “I haven’t heard of any pressure to rescind Mr. Trump’s offer to speak at CPAC.”

The anti-Trump super PAC Make America Awesome this week launched a petition calling on the ACU to rescind its invitation to Trump. If plans for Trump’s speech proceed, the group is planning some form of protest at the convention center hosting CPAC in the Maryland suburbs of Washington, according to Liz Mair, the GOP strategist behind the PAC.

Allowing Trump to speak “will do lasting and huge (yuge!) damage to the reputations of CPAC, ACU, individual ACU board members, the conservative movement, and indeed the GOP and America,” Mair wrote in an email. Her group’s petition notes that the ACU has in the past snubbed high-profile Republican politicians — notably including Arizona Sen. John McCain and New Jersey Gov. Chris Christie — for various deviations from conservative orthodoxy. McCain and Christie have both accepted invitations to speak in earlier years, and the petition asserts both men “are clearly more conservative” than Trump, who also has spoken at past CPACs, as have his rivals for the 2016 nomination — Rubio, Ted Cruz and Ohio Gov. John Kasich.

Trump personally accepted an invitation to speak at CPAC roughly 15 minutes after the invitation was emailed by ACU chairman Matt Schlapp. But other campaigns have been more deliberative about the gathering, which in recent years has developed a fringe, carnivalesque reputation, and that’s prompted some testiness between the campaigns and the ACU.

Kasich initially turned down the invitation, before agreeing to speak Friday, and Schlapp scolded Rubio for disrespecting grass-roots activists by refusing to commit to speaking at the conference until just last week.

Top officials on Rubio’s campaign in turn accused Schlapp of secretly favoring Trump and have leaked emails they say show Schlapp acting in bad faith in his negotiations to get Rubio to speak.

Schlapp, who responded by threatening to release emails that cast Rubio’s campaign in a bad light, has accused Rubio’s supporters of “Twitter bullying.”

While Schlapp has defended Trump against allegations of racism and has said the Manhattan mogul is “definitely” a “plausible president,” he has rejected suggestions that he’s supporting the GOP front-runner and has defended the decision to invite him to CPAC.

“I support having all 5 candidates for president at CPAC … Conservatives want to hear from them,” Schlapp wrote on Twitter this week in response to a series of tweets from Rubio’s deputy campaign manager, Rich Beeson, suggesting that Schlapp’s invitation to Trump was an affront to conservatism and Christianity, and a move toward a “bigoted & extremist” GOP.

Calling Trump “your guy,” Beeson tweeted at Schlapp: “There comes a time for choosing … and you have made your choice.”

Schlapp declined to comment on the exchange, though he tweeted at Beeson: “Rich, you and I have been friends for 16 years. I’m not your problem. Glad you are coming to CPAC.”

Beeson, who is known for lashing out at opponents, would not elaborate on why he believed Schlapp is supporting Trump.He deleted the tweets containing his direct allegations against Schlapp, though he said he didn’t regret making them. “I just thought I had too many and they were losing effectiveness,” he explained.

Rubio’s campaign declined to say whether it stood behind Beeson’s accusations about Schlapp or his suggestion that ACU should rescind its invitation to Trump.

While Rubio has spoken at CPAC in past years and has a high congressional-vote rating from the ACU, some of his advisers argued that the Florida senator should not attend the event, according to an operative with knowledge of the CPAC negotiations. Additionally, the operative said some on Rubio’s team were bitter that Schlapp publicly called out Rubio for dragging his feet for months on whether he planned to accept ACU’s invitation.

Rubio spokesman Alex Conant pointed out that Rubio has spoken at CPAC every year since 2010 “and we always planned to attend this year.” He added that “there was some unfortunate confusion about the schedule earlier but we're looking forward to Saturday's remarks,” and he said the campaign is encouraging its supporters to attend.

Ned Ryun, an ACU board member, said the group is “very happy that Sen. Rubio is coming. It would have been hard for him to say that he’s the best candidate to carry the mantle of Reagan conservatism, and then not come to talk to the conservative grass roots.”

Still, it would add insult to injury for Rubio, who was left clinging to political life after a dismal showing in this week’s Super Tuesday contests, if he attended CPAC only to lose to Cruz and Trump in the event’s fabled presidential straw poll. Rubio has tried to make the argument that he is the candidate who can beat Trump and Democratic front-runner Hillary Clinton. But CPAC’s straw poll tends to favor more conservative or libertarian candidates, such as Kentucky Sen. Rand Paul, who has since dropped out of the GOP presidential race.

Those donations were publicly revealed by the committees that donated the cash, which are required by federal election law to report their receipts and expenditures. Donations made by individuals like Trump, or corporations, to ACU are not subject to disclosure rules because ACU is registered under sections of the tax code ― 501(c)(3) and 501(c)(4) ― that allow groups it to shield their donors’ identities.

In addition to Trump’s $50,000 donation to ACU last year, he also gave multiple donations before that, according to sources familiar with ACU’s finances. That includes another $50,000 before the 2014 CPAC conference. At the time, some meeting organizers didn’t want to allow him to speak, because they argued he was “not a serious movement leader,” according to McKay Coppins’ 2015 book “The Wilderness: Deep Inside the Republican Party’s Combative, Contentious, Chaotic Quest to Take Back the White House.” After the check, Trump got a prime CPAC speaking slot, though ACU officials denied the two were related.

Walters, the ACU spokesman, told POLITICO, “Generally, we don’t talk about donors and who supports us.” He noted, though, that none of the 2016 candidates have been asked for donations this year, either personally or through PACs.

ACU has no special plans to respond to any potential protests against Trump, Walters said, explaining “we believe in the First Amendment and don’t believe in censorship.” But, he added, “if you disrupt the program, we will ask you to not do that. ... Our priority is to make sure our attendees can see the program. So you can speak your mind, but don’t disrupt the program.”

Walters passed along a statement from ACU that said: “We welcome all good conservatives to come to CPAC, vote for their preferred candidate in our straw poll and be a full participant in the discussion about the direction of our movement, and America. We believe that all conservatives have it in them to remember that we, in the end, all share a similar goal, and that is for us to stay focused on defeating the Left and rebuilding America.”